Combined Storm Overflows (CSOs)

Facts on CSOs

1. The CSO system performs a public health function to manage excess sewage overflows under extreme weather conditions. CSOs are permitted to operate at these times to relieve the sewerage network and prevent sewer flooding. Discharges through CSOs will be diluted and the bacterial loading impact from these will generally be minimal.


2. CSOs are regulated by Natural Resources Wales (NRW), the body that sets the permit conditions which must be met by CSOs. The number of spills from a CSO that are allowed is dependent upon the environmental sensitivity of the watercourse to which the discharge is made.


3. Dŵr Cymru Welsh Water (DCWW) has taken the most proactive position on CSO monitoring and their data is more extensive and of better quality than any of the other UK water companies. All CSOs in Wales were monitored by the end of 2020 (approx. 3000 assets). These data have been reported on Dŵr Cymru Welsh Water’s website by region and have been pro-actively shared with both Afonydd Cymru and the Rivers Trust. Welsh data is included on the Rivers Trust sewer map ( which shows the geographical distribution of CSO spills across Wales. Despite this, the data for 2020 clearly shows that there are a significant number of CSO spills across Wales, including those that are operating outside of their permit conditions.


Afonydd Cymru’s Position

1. We expect NRW to effectively regulate CSOs, and where a CSO exceeds the number of spills per annum – which are defined within their permit – to take the necessary enforcement action. We will also work with NRW to determine their response to the 2020 CSO dataset.

2. We will work with Dŵr Cymru Welsh Water on their AMP7 plans for CSO improvements and support increased investment to improve the CSO network. We have already established a procedure with the company to investigate and report individual CSO issues that are impacting the environment.

We support Welsh Water hotline for public to report CSO pollution.

Afonydd Cymru has examined the programme of Sewer Overflow Assessments and will identify the high spillers identified through this monitoring data; these are targeted around those CSOs which have the potential for the greatest environmental impact. In addition, both Afonydd Cymru and the Rivers Trust movement will actively contribute to the development of Drainage and Wastewater Management Plans in Wales.

3. We will press for more research into the environmental impacts from CSO discharges on our rivers, in terms of their effect on biodiversity, on bacterial loading as well as wider concerns around plastics and pharmaceuticals.

Afonydd Cymru continues to campaign for the environmental protection of our watercourses.

More Information

Afonydd Cymru’s Views on CSOs

Afonydd Cymru is concerned about all sources of pollution affecting our rivers in Wales. CSOs can cause pollution and increased microbial loading particularly after periods of heavy rainfall. Where this occurs, we would encourage reporting to DCWW on their Pollution Hotline 08000853968 as it is important that problematic CSOs are speedily rectified. However, from the evidence that we have examined, both from NRW’s monitoring data and our observations when carrying out River Habitat Surveys, there is little current evidence that CSOs are responsible for widespread deterioration in the overall water quality in Welsh rivers. We would, however, like to see more monitoring and evaluation of the environmental impact of CSO discharges particularly given the high spill frequency of certain CSOs. We are encouraged by DCWW’s proactive approach and willingness to work with us to address these problems through Drainage Water Management Plans (DWMPS) and the Storm Overflow Assessment Framework (SOAF) and through its investment programme (see below). Compared to most water companies in the UK, DCWW has been at the forefront of addressing the issue of CSOs and has openly published data on the performance of their CSOs.

The big question is whether in the 21st century we should persist with a sewerage system that discharges into the natural environment without full treatment? However, we are aware that to replace this system would cost many billions of pounds, create massive disruption particularly in urban areas and take many years to undertake. The funding would almost certainly come from increased water bills and the ability and willingness of the public to pay for this is unknown. However, in the meantime, we believe that the current approach of no new CSOs, rapid investigation of and enforcement against polluting CSOs and the implementation of DWMPs is the best way forward.

An additional issue to consider is the possible human health impacts of CSO discharges which are an increasing concern particularly in relation to wild swimmers, canoeists and anglers. The reality is that bacterial loading in rivers will come from many sources e.g. From livestock and septic tanks to name just two. We would very much support more research into the potential health impacts of CSOs, a system to warn water users when major CSO discharges are occurring and the development of designated Inland Bathing Waters for wild swimmers where water quality standards are set and regularly monitored.

Afonydd Cymru will continue to take a close interest in CSOs and their impacts on the health of our rivers and the people using them, however, our current highest priority is addressing agricultural pollution which we believe will turn around the deterioration in the health of our rivers most speedily.

Background Information: Combined Sewer Overflows in Wales

Afonydd Cymru is dedicated to improving the water quality and biodiversity of Welsh rivers. We are a solutions and evidence led organisation and we want to ensure that there is a clear understanding of the history of combined sewer overflows (CSOs), how these are operated and regulated, their environmental impact as well as how they can be improved.

What are CSOs?

CSOs were designed in Victorian times to prevent sewage backing up into people’s homes by allowing the sewerage system to spill into our rivers when its capacity is exceeded, a form of pressure valve.

These spills can contain sewage which has only been screened, however this does not necessarily mean that the environment is damaged as in most cases such spills occur under high flow conditions when dilution is high. Further information on CSOs and how these operate is available at:…/wastewater/combined-sewer-overflows

Who is responsible for the operation and regulation of CSOs?

Dwr Cymru Welsh Water (DCWW) is the body which operates the sewerage system in Wales and Natural Resources Wales (NRW) is the body responsible for regulating their use. All combined sewer overflows are consented and DCWW was the first water company in England and Wales to monitor all their CSOs. The spill frequency of all CSOs in Wales is also recorded on DCWW’s website. The attached map shows the distribution and spill frequency of CSOs in Wales.

How much is being invested to improve CSOs?

DCWW has spent more than £1 billion on improving CSOs since 1990 (this does not include maintenance costs). The following shows the investment made by DCWW during the subsequent Asset Management Periods (AMPs)

  • AMP3 (2000-2005) £471m in CSO improvements
  • AMP4 (2005-2010) £400m in CSO improvements
  • AMP5 (2010-2015) £47m in CSO improvements
  • AMP6 (2015-2020) £125m which includes investment in Loughor to reduce spills
  • AMP7 (2020-2025) planned investment £82m on CSO improvements (with an additional £25m on improvements to previously improved assets with Trigger Breaches so technically only restoring them to what they should be).

All this investment is raised from householder’s water bills.

DCWW is currently investigating the environmental impact of the most frequently spilling CSOs through the Storm Overflow Assessment Framework (SOAF) as part of a wider initiative known as Drainage Water Management Plans (DWMPs). These plans which will become statutory through the forthcoming Environment Bill are designed to take a long-term strategic approach to improving drainage systems in order to reduce flooding and pollution including through CSOs. Those CSOs causing failure to achieve ‘good status’ under the Water Framework Directive (WFD) will be prioritised.

An explanation of the regulatory framework for CSOs

All CSOs are consented by NRW and it is a requirement that all storm overflows comply with relevant design and water quality standards and cause no deterioration to the existing quality of the receiving water.

Currently, there is a presumption against permitting new storm overflows, unless as part of a wider scheme delivering a net improvement in water quality.

The compliance record of DCWW in relation to CSOs

In 2019, DCWW’s CSOs caused 15 Low level (Cat 3) incidents in England and 1 in Wales.

10 CSOs out of a total of 3000 were identified as non-compliant with their permit conditions. All breaches were found to be Category 3 (reasonably foreseeable to cause a Low/Minor impact to environment and/or property).

What enforcement has been taken in relation CSOs?

When a permitted CSO causes an incident a compliance assessment is carried out against the permit and a Compliance Assessment Report (CAR) is completed.

In 2019, NRW issued DCWW with 11 CARs for non-compliant CSOs.

In addition, 6 Warnings were issued to DCWW due to CSO permit non-compliances, as well as providing advice and guidance and requiring actions to be undertaken. One CAR is still under investigation.

What environmental impact CSOs have and how these impact on meeting Good Ecological status under the Water Framework Directive?

WFD investigations have identified 32 water bodies where intermittent sewage discharges are identified as part of the reason for not achieving Good Ecological Status. All these water bodies failed for multiple reasons. However, it should be noted that:

  • None of the failures are confirmed.
  • 8 are probable with 5 being considered a major pressure.
  • 24 are suspected with 5 being considered a major pressure.

SOAF investigations during AMP7 will provide further monitoring and evidence of the impacts of high spilling CSOs.

Water companies have been installing Event Duration Monitoring throughout AMP6. DCWW has installed monitors on all their CSOs (approx. 3000 assets, as of the end of March 2020).